Gunmen killed a police officer Friday in northern Egypt as supporters of the country's ousted president held scattered demonstrations that saw one person killed, authorities said.

The police officer had been riding a motorcycle in the Nile Delta city of Mansoura when gunmen on another motorbike opened fire on him, the Interior Ministry said in a statement on its official Facebook page.

It did not elaborate.

Meanwhile, security officials also said they dismantled a homemade bomb on a main bridge in the north of Cairo.

A series of bombings and targeted killings, mainly striking security forces and installations, have hit the country since the military overthrow of Islamist President Mohammed Morsi in July.

Authorities largely blame the attacks on Islamic militants loyal to the ousted president.

Violence initially surged last summer in the restive Sinai Peninsula before gradually spilling over into Nile Delta cities and the capital.

Meanwhile, hundreds of Morsi supporters took to the streets on Friday, the traditional Muslim day of prayer, to denounce the military-backed government and call on the release of their fellows detained during other demonstrations.

They've been regularly protesting Friday since Morsi's overthrow.

A health official said one person was killed during a clash between Morsi supporters and opponents in the Mediterranean city of Alexandra.

He spoke on condition of anonymity as he wasn't authorised to speak to journalists. During protests in Cairo, Suez and Alexandria, police fired tear gas to disperse demonstrators.

Egypt's military-backed interim government has cracked down hard on Islamists after Morsi's ouster, detaining thousands of them while putting the group's top leadership on trial.